Efficient and Durable Sodium, Chloride‐doped Iron Oxide‐Hydroxide Nanohybrid‐Promoted Capacitive Deionization of Saline Water via Synergetic Pseudocapacitive Process

Abstract Recently, the rational design and development of efficient faradaic deionization electrodes with high theoretical capacitance, natural abundance, and attractive conductivity have shown great promise for outstanding capacitive deionization (CDI)‐based desalination applications. Herein, the construction of novel FeOOH hybrid heterostructures with Na and Cl dopants (e.g., Na‐FeOOH and Cl‐FeOOH) via a robust hydrothermal strategy is reported, and an asymmetric CDI cell (Na‐FeOOH//Cl‐FeOOH) comprising Na‐FeOOH and Cl‐FeOOH working as the cathode and anode, respectively, is assembled. The multiple coupling effects of the specific structural features (e.g., enriched porosity, hierarchical pore alignment, and highly open crystalline framework), enhanced electrochemical conductivity, and optimized ion‐transfer property endow the FeOOH hybrid electrode with improved electrochemical performance. Impressively, the Na‐FeOOH//Cl‐FeOOH cell demonstrates a superior salt adsorption capacity (SACNaCl) of 35.12 mg g−1 in a 500 mg L−1 NaCl solution, a faster removal rate, and remarkable cycling stability. Moreover, the pseudocapacitive removal mechanism from the synergetic contribution of the Na‐FeOOH cathode and Cl‐FeOOH anode account for the significant desalination promotion of the Na‐FeOOH//Cl‐FeOOH cell.


S1. Experiments
Material characterization: Elemental composition and surface state of the Cl-FeOOH, Na-FeOOH, and FeOOH samples were detailly studied by X-ray photoelectron spectroscopy (XPS), which was implemented on a VG Scientific ESCALAB Mark II spectrometer, and the spectra were calibrated with the C-C peak of 284.8 eV. To further characterize the phase composition, X-ray diffraction (XRD) test was carried out with a Rigaku D/MAX-RB X-ray diffractometer using the Cu Kɑ (40 kV, 20 mA) radiation and a secondary beam graphite monochromator. The surface chemical composition was identified using Fourier-transform infra-red spectroscopy (FT-IR, Bruker VERTEX70, Germany) operated at room temperature. Morphology and microstructure of FeOOH hybrid samples were observed via field emission scanning electron microscopy (SEM, Hitachi S-4800) and transmission electron microscopy (TEM, FEI Tecnai F20). Raman spectra were obtained on an optical microscope with the excitation of 514.5 nm line from an Ar + ion laser (Spectra Physics). Nitrogen adsorption-desorption isotherms were provided by ASAP 2020 Brunauer-Emmett-Teller (BET) analyzer, and specific surface area and pore volume properties were calculated by the BET method along with the estimated pore size distribution via Barrett-Joyner-Halenda (BJH) model.
Electrochemical performance measurements: Cyclic voltammetry (CV) plot was tested on the electrochemical workstation (CHI 660E) in a standard three-electrode system consisting of prepared electrode, graphite paper, and saturated calomel electrode (SCE) as the working, counter, and reference electrodes, respectively, and the specific capacitance (C m , F g -1 ) was calculated by Equation S1.
where m, △V, and v indicated the active mass, chosen potential range, and running rate, respectively. The Q was the integrated CV curve area.
3 GCD measurements were conducted using an automatic LAND battery test instrument (Land CT2001A) to evaluate both capacitive property and cycling performance. Coulomb efficiency (η, %) was obtained by the ratio between t d and t c . The t c was the charging time, and t d suggested the discharging time.
The relationship between peak current (I) and scan rate (v) in the CV curves can be described as follows: where the b = 0.5 corresponded to the ion intercalation process, and b = 1 implied the capacitance-like behavior.
Capacity contribution can be calculated based on CV results: where v was the scan rate. k 1 v stood for the contribution of capacitive-controlled current, and k 2 v 1/2 reflected the contribution of diffusion-controlled current. [1][2] The electrochemical impedance spectroscopy (EIS) was performed in a frequency window of 10 mHz to 100 kHz under a voltage amplitude of 5 mV.
The diffusion coefficient (D Na + , cm 2 s -1 ) was calculated by Equation S5 and S6 where F was the Faraday constant, R was the gas constant, T was the absolute temperature, c was the Na + concentration, A was the electrode surface area, was the Warburg factor, and Z re was the real part impedance.
Desalination behavior measurements: Charge efficiency (Λ) was quantitatively determined according to Equation S7.
The energy consumption (E, Wh g -1 ) was obtained as follows.
where the v indicated a driven potential of 1.2 V, dt corresponded to the integrated value of the current transient vs. running time plot, and V (mL) was the rotational solution volume. C 0 and C (mg L -1 ) were initial and final concentrations, respectively.
5 Figure S1. High-resolution N1s XPS spectra of three as-fabricated samples with (a) and without (b) the PVP additive.